In electron beam lithography, an electron beam is directed at a target material coated with a photosensitive material to write a desired pattern on the target material. When such a pattern is drawn, pattern data including the coordinates of the vertices of a polygonal pattern and the lengths taken in the x- and y-directions are generated by CAD. The data are then converted into a form that can be used by an electron beam lithography system in writing a pattern. The lithography system deflects the electron beam and/or moves the target material according to the converted data. As a result, the desired pattern is drawn.
For example, in vector scan, an electron beam is deflected to a portion to be written within a given field whenever the target material is stepped in the x- and y- directions. In this technique, a chip pattern having a complex polygonal pattern to be written is divided into simple rectangles or trapezoids.
When the polygonal pattern is divided into rectangles or trapezoids, the smoothness of the original geometrical pattern is lost. For instance, when an undivided polygonal pattern P indicated by the solid line is divided into two trapezoids P.sub.1 and P.sub.2 as shown in FIG. 1, the division line indicated by the broken line intersects the hypotenuse at point C. The position of this intersection C does not always overlap any one of grids that are coordinate points expressing the geometrical pattern. The grids representing coordinate points are coordinate points of a minimum unit in creating data and are indicated by x. The intersection C is approximated by a near grid G that is a vertex of the trapezoids P.sub.1 and P.sub.2 forming elements of the original pattern. Therefore, if these rectangles or trapezoids are recombined, the original polygonal pattern is not always regained. Note that in FIG. 1, the trapezoids P.sub.1 and P.sub.2 produced by the division and indicated by the broken lines are shown to be slightly shifted from the undivided polygonal pattern P indicated by the solid line for ease of understanding.
Also, once a pattern is divided into elements, it is impossible to write the pattern across the adjacent elements. Overlap writing is done in drawing each element of the pattern. In particular, whenever a shot of beam is made, the junction between two adjacently shot portions is shifted. Thus, the successively shot portions are written smoothly. However, smoothness cannot be achieved across the boundary line between the adjacent elements.
A chip pattern consisting of a polygon is divided into rectangles or trapezoids by the division method shown in FIGS. 2(a) and 2(b). Specifically, the pattern is divided by line segments that start at the vertices of the polygon and extend parallel to the x- or y-axis on the two-dimensional plane representing the chip pattern. FIG. 2(a) gives an example in which the pattern is divided by line segments parallel to the y-axis. FIG. 2(b) shows an example in which the pattern is divided by line segments parallel to the y-axis. Where this method is adopted, very small elements of quite small heights or widths may be produced. This problem is serious in the case of a variable-shaping beam electron beam lithography system.
In the variable-shaping beam electron beam lithography system, two slit plates are positioned along the optical axis. A deflector is disposed between the two slit plates to appropriately deflect the electron beam passed through the upper slit. Thus, the electron beam of the desired cross section is made to pass through the lower slit and bombard the target material. When such very small elements are written, the beam passing through the lower slit (shaped beam) becomes very small. This results in a very great decrease in the beam current density. As a result, the photosensitive material applied on top of the target material cannot be sufficiently activated. That is, if the elements of a polygon contain very small elements, the pattern cannot be drawn smoothly across the boundary between the adjacent elements.